The present invention relates generally to the on-line measurement of properties of a translucent moving web for process control and, more particularly, to the on-line measurement of color, reflectance and opacity of a moving web of paper as it is produced on a paper machine.
During the manufacture of paper, it is desirable to monitor characteristics such as color and opacity of paper as it is produced and to either continuously or periodically adjust process parameters to maintain desired characteristics of the paper being produced. Measurements of color and opacity are highly interrelated. Thus, measurements of color or reflectivity are sensitive to changes in opacity.
It is known to address the problem of color or reflectivity measurements being dependent upon opacity by backing the paper web with alternate black and white tiles and using the Kubelka-Munk formulas to estimate the reflectance of an infinitely thick stack of paper. Alternately, the paper web may be backed with a tile which matches the target reflectivity. As examples, De Remigis U.S. Pat. Nos. 3,936,189 and 4,015,904 disclose a stationary optical shoe assembly with white and black standards over which a web moves for measurement of color, opacity and brightness. A similar system is disclosed in Lodzinski et al U.S. Pat. No. 3,992,100 and Lodzinski U.S. Pat. No. 4,019,819.
The practice of using stationary or traversing flat black and white backgrounds behind the sheet material is well known to increase the likelihood of breakage of a sheet material such as paper as it is made on a paper machine. Contact between the paper and a flat stationary object also has the potential for producing scratch marks on the sheet material. Moreover, it is difficult to achieve complete contact between a moving web of paper and a stationary tile.
Accordingly, it is also known to employ backing tiles which are separated from the sheet material by an air gap. As examples, Howarth U.S. Pat. No. 4,319,847 discloses a system wherein a stream of air is employed to maintain a relatively constant spacing between a moving web, a standardization member below the web and a sensing head above the web. Similar systems with spacing are disclosed in Howard U.S. Pat. No. 3,455,637; and in Howarth et al U.S. Pat. No. 4,715,715. Although such practices can avoid adverse effects on productivity and sheet quality, several other problems are introduced. For example, the presence of an air gap changes the appearance of the translucent sheet material when viewed by the sensor over the backing. Formulas such as the Kubelka-Munk formulas for predicting the appearance of a thick sample are no longer valid. Such practices using a backing or backings not in contact with the sheet material are also subject to the possibility of fluctuations in the position of the material relative to the backing. The position is changed by variations in the thickness of the air gap.
An alternative approach uses a thick stack of the same sheet material as a backing behind the moving web of sheet material. This approach can suffer from a number of adverse effects, including: (a) fading of the material due to long exposure to light and environmental conditions in general; (b) wear, abrasion and mechanical damage of the backing if it is in direct contact with the moving web of sheet material; (c) error in reflectance measurement if the backing material is protected by a window; (d) an excessive amount of time and effort required to prepare different backings for each color of translucent product to be produced; (e) potential web damage due to contact of the moving web with a stationary stack of backing paper; (f) influence of the gap variation on the sheet reflectivity measurement; and (g) complexity of changing the backing with changes in product grade.
To solve the problems of fading, wear or both of the backing material, backing of a different material, such as ceramic, may be employed. However, such an approach is well known to require a greater amount of effort in preparing the backings and in matching the color of the sheet material to be measured or produced.
If no backing at all is employed behind the area of view on the sheet material, the measured reflectance of the material becomes a function of its opacity. This dependence of reflectance measurements on opacity is a source of major error of the color measurement, since the opacity varies for a number of reasons. For example, for paper there may be a change in thickness or basis weight of the sheet, filler content, hardwood-to-softwood ratio, sizing, and the like.
In this context, another approach is to separately measure reflectance of light from and transmission of light through the translucent sheet material. However, this approach differs fundamentally from the technique specified in the definition of opacity (TAPPI method T425 discussed hereinbelow), and correlation between the two methods is never perfect.